Vehicle engines may be configured to be shut-off during idle conditions, such as when the vehicle comes to a stop while a brake is applied, and restarted once the brake is released (e.g., a stop/start system). Such stop/start systems enable fuel savings, reduction in exhaust emissions, reduction in noise, and the like. Fuel consumption may be further reduced by shutting down the engine before braking, for example, while the vehicle is moving during extended coasting periods.
However, the inventors have recognized an issue with such systems. Frequent switching between engine idle-stop and engine restart operations may lead to objectionable noise and clunks that affect drive feel due to the transition between engine stopped and engine spinning conditions. For example, lash in various transmission components, such as gears, chain drives, differentials, etc., can cause increased noise, vibration, and harshness (NVH), due to engagement and disengagement that occurs. As such, these may reduce drive feel as well as customer satisfaction. Furthermore, repeated clunks and related torsional stress may degrade transmission or driveline components (e.g., transmission gears, clutches, etc.) over time.
Thus in one example, some of the above issues may be at least partly addressed by a method of operating a vehicle engine having a torque converter and an automatic transmission, each with a clutch. One example embodiment comprises, during a vehicle moving condition, spinning down the engine to rest, and during a starter-assisted restart from the rest, and while the torque converter clutch is disengaged, adjusting a degree of transmission clutch engagement based on torque converter output speed relative to torque converter input speed.
In one example, a vehicle may be coasting with the accelerator pedal and the brake pedal not depressed, and a vehicle speed being above a threshold. In response to the coasting condition, the engine may be shutdown, for example, by shutting off a fuel supply and spark to the engine. While the engine spins down to rest, a torque converter clutch (e.g., torque converter lock-up clutch) may be disengaged. Further, a clutch pressure of a vehicle transmission clutch (e.g., forward clutch) may be reduced to a near stroke level (e.g., at or just below the stroke level) during the engine spin-down. In this way, the clutch may be kept pre-stroked by maintaining the clutch pressure near a stroke level until a subsequent restart from engine rest.
During a subsequent restart, while the vehicle is still moving, and while the torque converter clutch is still disengaged, the engine may be started using starter assist by activating a starter motor. Further, a degree of engagement of the transmission clutch may be adjusted (e.g., increased) based on a torque converter output speed relative to a torque converter input speed. In one example, when the torque converter input speed (as inferred from the engine speed, for example) is lower than the torque converter output speed (as inferred from a transmission input shaft speed, for example) indicating that the engine is not yet applying positive torque to the driveline, a clutch pressure of the transmission forward clutch may be increased at a first lower rate, and/or to a first lower amount of clutch pressure. The first amount and/or rate may be based on a difference between the speeds when the torque converter input speed is lower than the torque converter output speed. In comparison, once the torque converter input speed exceeds than the torque converter output speed and the engine is now applying positive torque through the driveline, the clutch pressure may be increased at a second higher rate, and/or to a second higher amount of clutch pressure. The second amount and/or rate may be based on a difference between the speeds when the torque converter input speed is higher than the torque converter output speed. In this way, the degree of torque applied during the transition through the lash region may be kept relatively low until after the lash transition is completed (as indicated by the torque converter speeds) to reduce driveline clunk. Then, once the transition is completed, the clutch pressure may be applied to a sufficient level to provide the desired torque to propel the vehicle.
It will be appreciated that while the degree of transmission clutch engagement may be adjusted based on a difference of speeds across the torque converter, other approaches are possible. For example, the adjustment may be based on a ratio (or alternate function) of speeds across the torque converter.
In this way, an engine may be restarted with starter motor assistance while the vehicle is still moving, with reduced torque disturbances in the driveline. By keeping a transmission clutch pre-stroked during an engine shutdown, delays in clutch engagement during a subsequent restart may be reduced. By reducing the torque capacity of a transmission clutch during an engine shutdown, and then increasing the torque capacity of the clutch based on an engine speed relative to a transmission input shaft speed, a smoother transition through the lash region of the transmission may be enabled. By improving overall engine torque control during an engine restart, torque disturbances and audible clunks may be reduced, thereby substantially improving the quality of engine restarts.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.